Pinpoint precision A detailed analysis of images collected by a European spacecraft has helped astronomers pinpoint the exact orbit of the tiny Martian moon Deimos.

The work may also shed light on the history of the Martian moons.

The orbit of Mars' largest moon Phobos is well known and has been calculated to an accuracy of less than a kilometre. However, the details of the smaller and more distant moon Deimos have been far less understood.

To fill in the missing peices, scientists developed a new technique to study its orbit by comparing a range of images of the moon taken by the European Space Agency's Mars Express spacecraft.

Between July 2005 and July 2011, the spacecraft made 50 close approaches to Deimos, passing within 14,000 kilometres of the satellite, taking 136 images at different places along Deimos' orbit.

The team used two star trackers and three laser gyroscopes to determine the exact position, attitude and orientation of the spacecraft as Deimos passed between it and background stars, used as reference points.

"From 50 sets of observations, we fortuitously had nine in which stars were sufficiently bright to be seen in all images," says study co-author Andreas Pasewaldt, a PhD student at the Institute of Planetary Research in Berlin.

"We obtained a set of spacecraft-centered Deimos coordinates with accuracies between 0.6 and 3.6 kilometres."

Captured asteroids?

Both moons were first seen orbiting Mars 135 years ago.

Phobos is relatively close to the red planet and is strongly affected by its gravity, which will eventually cause it to crash onto the Martian surface or break apart in orbit creating a debris ring.

In contrast, Deimos is far enough from Mars to be slowly spiraling outwards.

European Space Agency's Mars Express project scientist Olivier Witasse says there are still questions about the origin of both moons. This latest study may help in solving that mystery.

"It is unclear whether they are asteroids that were captured by Mars or whether they coalesced from a ring of material that formed around the planet after a large object collided with Mars, although the latter scenario seems to be favored in recent years," he says.

Witasse adds that "better orbital models are also important for future satellite missions, such as automated sample returns currently being studied at ESA.

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